efl_input: remove the API of efl_input_instance_get

efl_input: remove the API of efl_input_instance_get

there is basically no reason for this API. You can only use the API when
you know the class, when you know the class you can also just know the
function to call to get this API.
The reason this API needs to go is that we don't want to use
polymorphism on class-functions.

ref T7675

Reviewed-by: Xavi Artigas <xavierartigas@yahoo.es>
Differential Revision: https://phab.enlightenment.org/D7900